1. Field of the Invention
The present invention relates to a slip frequency type vector control apparatus for an induction motor, which uses a neural network capable of automatically meeting changes in rotor resistance or mutual inductance of an induction motor.
2. Description of the Related Art
Conventionally, a vector control system has been often used as a variable speed driving system for an induction motor. The vector control system is designed to handle an induction motor in the same manner as a DC motor. The system separates the rotor side into a torque-axis component and a flux-axis component and controls the individual axial components.
The vector control system is classified into a magnetic field orientation type in which the rotor flux is used as a vector quantity to control the primary current and a slip frequency type in which the flux vector is controlled by arithmetic operations on the basis of induction motor parameters.
FIG. 9 shows functional blocks of a slip frequency type vector control apparatus, in which reference numeral 2 denotes an induction motor as an object to be controlled.
In this vector control apparatus, an externally output rotor flux command value .phi.2* is input to an excitation current calculating unit 1 and a slip frequency calculating unit 5, and a speed command value .omega.r*, which is similarly externally output, is input to the comparator C1. Three-phase input currents ia, ib, and ic input to the induction motor 2 are detected by current detectors CT1 to CT3, respectively, and a rotational speed .omega.r of the induction motor 2 is detected by a rotational speed detector 3.
The excitation current calculating unit 1 calculates an excitation current command value id* from the rotor flux command value .phi.2* on the basis of equation (1) below and supplies the obtained id* to a comparator C2: EQU id*=.phi.2*/M+[L2/(M.multidot.R2)].multidot.d.phi.2*/dt (1)
where M is the mutual inductance of the induction motor 2, L2 is the rotor inductance of the induction motor 2, and R2 is the rotor resistance of the induction motor 2.
The comparator C1 calculates the deviation between the speed command value .omega.r* and the rotational speed .omega.r and supplies the obtained deviation to a control compensating circuit 4. The control compensating circuit 4 amplifies the deviation and supplies the result as a torque current command value iq* to a slip frequency calculating unit 5 and a comparator C3.
The slip frequency calculating unit 5 calculates a slip frequency command value .omega.s* from the torque current command value iq* on the basis of equation (2) below and supplies the obtained .omega.s* to an adder 6: EQU .omega.s*=(M.multidot.R2/L2).multidot.iq*/.phi.2* (2)
The adder 6 adds the slip frequency command value .omega.s* and the rotational speed .omega.r detected by the rotational speed detector 3 to obtain a primary angular frequency .omega.e and supplies the obtained primary angular frequency .omega.e to an integrating circuit 7. The integrating circuit 7 calculates a phase angle .theta. by integrating the primary angular frequency .omega.e and supplies the phase angle .theta. to a ROM 8.
The ROM 8, which constitutes a memory table, generates two-phase unit sinusoidal signals sin.theta. and cos.theta. in response to the input phase angle .theta. and supplies these unit sinusoidal signals sin.theta. and cos.theta. to coordinate transforming circuits 9 and 10.
The coordinate transforming circuit 9 transforms the thee-phase currents ia, ib, and ic detected by the current detectors CT1, CT2, and CT3, respectively, into an excitation current (direct-axis current) id and a torque current (quadrature-axis current) iq of a dq coordinate system (rotating coordinate system) on the basis of equations (3) and (4) below: EQU id=cos .theta..multidot.i.alpha.-sin .theta..multidot.i.beta.(3) EQU iq=sin .theta..multidot.i.alpha.+cos .theta..multidot.i.beta.(4)
where ##EQU1##
The comparator C2, on the other hand, compares the excitation current command value id* with the detected excitation current value id to obtain a deviation .epsilon.d=id*-id. A control compensating circuit 11 amplifies the deviation .epsilon.d=id*-id and supplies an amplified output ed* to the coordinate transforming circuit 10. The output ed* serves as the d-axis component of a voltage command value. Similarly, the comparator C3 compares the torque current command value iq* with the torque current iq as the quadrature-axis component to obtain a deviation .epsilon.q=iq*-iq. A control compensating circuit 12 amplifies the deviation .epsilon.q=iq*-iq and supplies the amplified output eq* to the coordinate transforming circuit 10. This output eq* serves as the q-axis component of the voltage command value.
The coordinate transforming circuit 10 transforms the voltage command values ed* and eq* of the d and q axes into three-phase voltage command values ea*, eb*, and ec* on the basis of equations (5) and (6) below: ##EQU2## where e.alpha.*=cos.theta..multidot.ed*+sin.theta..multidot.eq*
e.beta.*=-sin.theta..multidot.ed*+cos.theta..eq* PA1 a parameter detector for detecting an input current and an input voltage to an induction motor as control parameters of the induction motor; PA1 a calculating unit for calculating a rotor flux and a torque current of the induction motor on the basis of the control parameters; PA1 a neural network for receiving a rotor flux command value and a torque current command value for the induction motor and rotor flux and torque current values calculated by the calculating unit, or the rotor flux command value, the torque current command value, the calculated rotor flux value, the calculated torque current value, and a detected torque current value, performing learning on the basis of a back-propagation law using signals output in correspondence with the inputs, and outputting an excitation current command value and a slip frequency command value; and PA1 a vector control unit for detecting an actual excitation current and an actual torque current of the induction motor on the basis of the slip frequency command value output from the neural network and the control parameters, and controlling the induction motor in accordance with a deviation between a detected actual excitation current value and the excitation current command value and a deviation between a detected actual torque current value and the torque current command value.
A power converter 13 for outputting three-phase AC power having a variable voltage and a variable frequency to the induction motor 2 is constituted by, e.g., a pulse width modulation control (PWM) inverter or a cycloconverter. The power converter 13 generates voltages proportional to the three-phase voltage command values ea*, eb* and ec* input from the coordinate transforming circuit 10 and controls armature currents ia, ib, and ic of the induction motor 2.
The conventional slip frequency type vector control apparatus for an induction motor having the above arrangement performs control such that the detected excitation current value id and the detected torque current value iq coincide with their respective command values, thereby achieving characteristics equivalent to those of a DC motor.
The above conventional vector control apparatus, however, has a drawback in that the operation accuracies of the excitation current calculating unit 1 and the slip frequency calculating unit 5 are decreased if errors are included in the constants (M, R2, and L2), and this degrades the output characteristics of the induction motor.
The constants (M, R2, and L2) of the induction motor 2 change in accordance with the type or rated output of the induction motor, so it is difficult to confirm their correct values. Although arithmetic operations are normally performed using design values, whether the design values of the constants correctly coincide with their actual values is not confirmed. Therefore, it is doubtful whether accurate vector control is realized.
The actual circumstances are that a skilled engineer performs a test operation and adjusts the constants (M, R2, and L2) to values supposed to be optimal while measuring the stationary characteristics. This adjustment is time- and cost-consuming and can be performed only roughly for a general-purpose motor having a large number of induction motors. This poses problems such as degradation in output characteristics of the motor.
The rotor resistance R2 of the induction motor changes in accordance with the temperature change in the rotor, and the mutual inductance M or the rotor inductance L2 of the induction motor changes in accordance with saturation of the core. Therefore, even if correct values are initially set, there is a possibility that the angle between the vectors of the magnetic flux .phi.2 and the torque current iq is not maintained at a right angle any longer due to changes in constants during the operation, thereby decreasing the generated torque of the induction motor.